Ever since 1776, the casual genius of America has been to inspire a cross-section of the majority with hope and energy, irrespective of party affiliation. The new state began with a brilliant advertisement for a better future – "life, liberty and the pursuit of happiness" – to which the average citizen could subscribe with patriotic zest. God and the flag were always important, but the idea of self-advancement, both for oneself and the greater good, was braided into the DNA of the constitution. Middle America was born.

Middle America is both a state of mind and a habit of being. It can be found on the prairies of the midwest; in the abattoirs of Chicago; in the snows of Iowa and North Dakota; in the Texas panhandle and the steamy bayous of Louisiana; in the sierras of California and the woods of Maine. Middle America is the subject of some great American classics from In Cold Blood to The Adventures of Tom Sawyer and its symbol is the white picket fence. Norman Rockwell is its artist.

Some of the most notable American presidents were patrician grandees such as FD Roosevelt, Woodrow Wilson and even John F Kennedy. But the true expression of Middle America comes from names such as Nixon, Reagan and Clinton, born and raised far from the privilege of the east coast. In art and entertainment, culture and society, the most popular Americans are the ones who articulate the beliefs and values of Middle America.

The astonishing resilience of the American middle class, a far wider sector of the community than in Britain, has been a key to US growth and confidence. The mix of fierce individualism with the spirit of the three musketeers ("All for one and one for all") has kept American society internally dynamic and externally on top of its game.

So last week's news that, for the first time in decades, America's middle class is in relative decline vis à vis its international rivals, is more than just a statistic. The figures reported by the New York Times have put the writing on the wall for Middle America in the starkest possible terms.

For the first time in 40 years, a person on median income in America is less well-off than a Canadian equivalent. The idea that Canada, the butt of late-night satire, should have overtaken the US as the best place in the world to be middle-class has unleashed a wave of doom-mongering.

"The idea that the median American has so much more income than the middle class in all other parts of the world is not true these days," said Lawrence Katz, a Harvard economist.

"In 1960, we were massively richer than anyone else. In 1980, we were richer. In the 1990s, we were still richer." But, said Professor Katz, this is no longer the case.

Three factors are influencing the weak income performance of the US. First, the education boom in the industrialising world, especially India and China, has made it far harder for the American economy to maintain its share of highly skilled, well-paid jobs.

Second, American companies' distribution of their bounty has been less generous than elsewhere. Top executives make much more money in the US compared with other wealthy countries. Finally, the US tax system tends to favour the rich, not the middle-class.

This data comes at a bad time for white, middle-class America. Squeezed in their wallets, shaken by the credit crunch and dismayed by the implosion of traditional Republicanism, the future – the one thing they have always believed in – looks bleak.

These figures tell us in a nutshell why the red states are so angry and why President Obama used his recent state of the union address to accuse Congress of failing middle-class earners.

There is, however, some hope to be found in the republic's chequered history. Just over 100 years ago – in the era of the robber barons – middle-class America was similarly squeezed.

More by luck than design, an ebullient populist with a taste for poker and big-game hunting emerged to renew the American challenge and lead his country into the so-called Progressive era. Middle America adored Theodore Roosevelt. They even named the teddy bear after him.

The best riposte to the challenge to middle-income America might be Middle America's capacity to surprise and confound its critics, as well as its best hope in a difficult future.

The United States has survived for nearly 250 years. We are not yet ready to write it off.

Michael Cohen: For years, the US enjoyed unrivalled prosperity - and its middle classes were the envy of the world. No longer. Last week, a study of decades of economic data concluded that the US had fallen behind other nations, apart, that is, from the super-rich